Power transistors, such as power MOSFETs (metal oxide semiconductor field effect transistors) or power IGBTs (insulated gate bipolar transistors), are widely used as electronic switches for switching electric loads, such as motors, actors, lamps, or the like. In many applications, load paths of two power transistors are connected in series between terminals for positive and negative supply potentials so as to form a half-bridge circuit, where the load is coupled to an output of the half-bridge. In a half-bridge circuit the transistor connected between output and a terminal for a negative supply potential is referred to as low-side transistor (low side switch), while the transistor connected between a terminal for the positive supply potential and the output is referred to as high-side transistor (high-side switch).
A power transistor is a voltage controlled device that can be controlled by a drive signal (drive voltage) received by a control terminal, which in a MOSFET or an IGBT is a gate terminal. While the low-side transistor can be controlled using a drive signal that is referenced to the negative supply potential, driving the high-side transistor requires a drive signal that is either referenced to the positive supply potential or to the electrical potential at the output terminal, where the electrical potential at the output terminal may vary between the negative supply potential and the positive supply potential, dependent on the switching state of the half-bridge. For driving the high-side transistor and the low-side transistor it is desirable to use a control circuit that generates control signals referenced to the negative supply potential. While the control signal for the low-side switch may be directly used for driving the low-side transistor, a level shifter may be required for shifting a signal level of the control signal for the high-side transistor to a suitable signal level for driving the high-side transistor or to a signal level suitable to be processed by a drive circuit for the high-side transistor.
A level shifter, however, may require a high voltage device, such as a further transistor, that has a voltage blocking capability similar to the voltage blocking capability of the low-side transistor.
In order to reduce manufacturing costs and to reduce the size there is a need to implement a power transistor and a high voltage device in a common semiconductor body.